Pressure operating dispensing containers which utilize a piston longitudinally slidable within the container are known in the art. These pressurized containers are used to dispense a variety of different materials of varying viscosities. The containers generally include a cylindrical can closed at one end and are provided with a dispensing nozzle having a valve for controlled discharge of a product contained therein.
The piston is received within the container and serves to separate the container into two chambers. The product to be dispensed typically occupies the upper chamber, above the piston. A pressurized fluid which acts as a propellant, occupies the lower chamber, below the piston. The piston is generally in the form of an inverted cup and has a upper surface and an annular skirt or side wall which extends down from the upper surface. The upper surface acts as a barrier wall to separate the product and propellant. The annular side wall of the piston stabilizes and positions the piston in the container and provides a surface which rides on the inner wall of the container.
The product to be dispensed is loaded into the upper chamber of the container under pressure. The loading is a three stage operation. During the first stage, known as the fill stage, the product is introduced into the can above the top of the piston. During the second stage, known as the pressure stage, a pressure differential is created above and below the piston to force some of the product down around the periphery of the piston, between the piston sidewall and the container. During the third stage, known as the pushup stage, the piston is pushed toward the top of the container. This pushup stage eliminates the air in the head space on the top of the product and also causes product to seep down around the periphery of the piston. After the loading of the product into the chambers is completed, propellant is loaded into the lower chamber under pressure. In use, when the valve at the top of the container is opened, the propellant pushes the piston toward the top of the container, forcing the product to exit the container through the nozzle.
After the container is loaded, the piston must be able to maintain a seal between the piston sidewall and the container surface. It must also minimize secondary permeation which is the diffusion of propellant around the piston at the propellant-product interface. This secondary permeation allows propellant and product to mix and thus decreases product shelf life and may otherwise adversely affect the product. Further, during the dispensing, it is important to minimize the bypass of propellant around the piston skirt into the product.
The piston skirt length is a function of container diameter. Although a piston which provides little clearance between itself and the container inner wall decreases secondary permeation, this type of fit increases bypass. As the piston diameter approaches that of the container, thereby deceasing clearance, the likelihood of secondary permeation around the piston lessens. Further, to decrease this secondary permeation, the longer the length of a tight fitting piston, the better. However, a piston which provides little clearance over a distance also increases resistance to movement. This increased resistance to movement results in increased bypass when the container valve is first opened. Accordingly, the most effective piston is one which has a diameter capable of minimizing secondary permeation without concomitantly creating a bypass problem within the confines of the piston length necessitated by the particular can.
In U.S. Pat. No. 4,913,323, a stepped piston is described which does not deform, tilt or shift when the product is loaded into a container at high speed and which facilitates even distribution of product between the piston sidewall and the container. However, in some applications, such as when dispensing liquids and products of low viscosity, it is possible for a small portion of the liquid to pass the piston. This can be a problem, particularly when dispensing two part products where the two ingredients are separated in the can and mixed together in a particular ratio. The first part being a liquid could run past the piston so that when the second part is added the ratio would be altered and the result when discharging the product would not always be consistent, or it may result in an incomplete reaction of the ingredients.